This application relates to optical communications.
Optical communications use an optical modulator to modulate an optical carrier beam to carry digital bits for transmission over an optical link. An optical communication system can use optical wavelength division multiplexing (WDM) to transmit multiple optical carriers modulated to carry different optical data channels through a single fiber.
The performance of optical transmission can be characterized by various parameters, such as the optical signal to noise ratio (OSNR), the data bit error rate (BER) and the data bit rate per wavelength or data spectral efficiency. The signal quality of an optical WDM signal may be degraded by various effects in the optical transmission such as optical attenuation effects in fiber and optical dispersion effects including chromatic dispersion (CD), polarization mode dispersion (PMD) and polarization dependent loss (PDL) in fiber. Some techniques to mitigate degradation of optical signals in transmission use optical compensation devices in the optical transmission paths such as optical amplifiers against signal attenuation and dispersion compensation devices. Other techniques use various signal modulation techniques to generate modulated data formats that can tolerate signal degradation effects in optical transmission such as the fiber dispersion.